Government regulations concerning the release into the atmosphere of various exhaust emission constituents from automotive vehicles are becoming increasingly more stringent. As the regulations relating to emissions of oxides of nitrogen, carbon monoxide, and unburned hydrocarbons become more stringent, it is necessary to control the engine combustion process to avoid unnecessary instabilities and thus prevent formation of undesirable exhaust emissions.
Evaporative emission control is an important consideration in automotive design and necessitates that fuel vapor arising from the engine fuel system be drawn into the engine and burned. Because the fuel vapor can be combusted by the engine, an excessive flow of vapor may cause combustion instability, or perhaps even engine roughness or stalling.
U.S. Pat. No. 5,460,143 discloses an evaporative emissions control system in which a pressure transducer prevents purging of a carbon canister in the event that the fuel tank pressure falls to a negative value. U.S. Pat. No. 5,816,223 discloses a system in which purging is controlled not only when the tank pressure becomes negative, but in response to rapid fluctuations in the tank pressure whether at a positive or negative pressure. Rapid fluctuations may cause the air and fuel vapor entering the engine from the purge line of a carbon evaporative emission control canister to alter the combustion process.
Some fuel system vapor storage purge strategies rely on purge control valves that regulate a constant purge air/vapor mixture flow rate entering the engine for combustion. Constant flow regulation is attempted for vacuum levels ranging from very high to only a few inches of mercury below which the valve flow rate drops off. Under equilibrium conditions, fuel tank vacuum is equal to vapor storage canister system flow restriction. Vapor storage canister system flow restriction is a function of purge air flow through the system.
When the manifold vacuum falls below the constant purge flow vacuum levels, such as when the throttle is depressed for more engine power, significant purge flow can be lost. This loss in purge flow results in vapor storage canister flow restriction levels decreasing which, in turn, decreases the fuel tank vacuum levels. The tank vacuum levels decrease by drawing air into, or generating vapor within, the fuel tank vapor space to equalize system vacuums.
When the manifold vacuum increases, purge flow increases which creates higher vapor storage canister system flow restrictions. Fuel vapor mass must be drawn from the fuel tank vapor space in order to equalize the system vacuum levels. If a sufficiently large enough vapor mass is drawn from the fuel tank, undesirable rich engine air/fuel ratio conditions are created.